Previously, thin films of metal sulfide such as zinc sulfide, cadmium sulfide and lead sulfide, which are used for various devices such as sensors, optoelectric transducers, and display devices, were prepared principally using a vacuum method such as vacuum deposition or sputtering. However, the vacuum method has the disadvantages of high machinery cost and of low productivity, and therefore, a process for preparing inexpensive thin films of metal sulfide with high performance has been desired.
As methods to eliminate the above-discussed disadvantages, the following processes have been proposed, including the so-called spray method in which an inorganic metal salt and a soluble salt containing sulfur or selenium are sprayed on a substrate heated to an elevated temperature, as disclosed in the specification of U.S. Pat No. 3,148,084 (Sep. 8, 1964); a process for preparing thin films by the reaction of different compounds containing, respectively, a metal and sulfur (one of the compounds may be a gas), as disclosed typically in the specification of French Patent No. 1,297,777 (May 28, 1962); and a process by the application and thermal decomposition of a solution of complexes consisting of an organic ammonium salt and a sulfur compound, as exemplified in U.S. Pat No. 2,905,574 (Sep. 22, 1959).
The inventors of the present application found that a uniform thin film of metal sulfide with excellent characteristics can be prepared by applying to a substrate, an organic compound containing at least one metal-sulfur, bond in the structure thereof, and then thermally decomposing the compound in an inert atmosphere, as disclosed in the Japanese Laid-Open Patent Publication No. 61-166979 (Jul. 28, 1986), corresponding to a patent application which was already filed and laid open as a method to eliminate the disadvantages of the conventional inventions.
However, the above-mentioned conventional processes for preparing metal sulfide thin films have disadvantages, and therefore, it is difficult in many cases to use the obtained film as a thin film for electronics which is one objective of the present invention. That is, the spray method has the following problems: the uniformity of the films prepared is poor and the characteristics of the films tend to fluctuate; in cases where thin films are obtained by the reaction of two different compounds, uniform thin films can not readily be obtained, or the use of a special atmosphere and sometimes of highly toxic gases is essential.
Moreover, in the process for preparing metal sulfide thin films according to the above-mentioned invention by the present inventors, there is the problem that it may be difficult to use the film in applications where an electric field is applied to the film, because the film has a tendency to become porous in the ultrafine structure, although the uniformity of the film is excellent.